This invention relates generally to semiconductor lasers and, more particularly, to relatively high-power semiconductor lasers formed by coupling multiple semiconductor laser elements together.
Relatively high-power laser light sources are required in many optical communication systems for transmitting signals over long distances with high signal-to-noise ratios. Semiconductor injection lasers are particularly well suited for these types of systems because of their small size and reliability.
A semiconductor injection laser is a laser device in which a forward bias voltage is applied across a junction formed between an n-doped and a p-doped semiconductor layer. Excess electrons from the n-doped layer and excess holes from the p-doped layer are injected into an active region of the p-n junction by the bias voltage, where the excess electrons and holes recombine. At low current levels, the electrons recombine with the holes to produce spontaneous emission of photons in all directions in the active region. At higher current levels, the excess carrier density becomes high enough to produce an inverted population, yielding a positive gain. Stimulated emission occurs and a monochromatic, highly directional beam of light is emitted from the active region. The active region is bounded at opposite ends by cleaved crystal facets, one being a highly reflective surface and the other being a partially-reflecting surface through which the beam emerges. The active region is also bounded by etched side surfaces, to prevent emission in the lateral direction.
The power output of a single semiconductor injection laser is rather small and, therefore, is inadequate for most types of optical communication systems. The power output of an injection laser is limited by its power density and by the finite cross section of its active region. Operating an injection laser beyond its power density limit results in catastrophic damage or at least serious degradation in performance, principally due to pitting of the crystal facets or due to the formation of dark line defects.
Coupling multiple semiconductor injection lasers together in an array provides for greatly increased power output levels. Individual injection lasers can be optically coupled using several different techniques. One technique is to evanescently couple the injection lasers together by positioning the lasers in a parallel fashion in close proximity to each other, causing the optical waves produced by adjacent lasers to overlap. Another technique is to directly couple the injection lasers together through the use of Y-junction waveguides. Although these techniques significantly boost power output levels, only a limited number of lasers can be coupled together using these techniques because of amplified spontaneous emission, thermal effects and waveguide breakdown. Accordingly, there has been a need for a technique for integrating large numbers of semiconductor injection lasers, preferably over a full wafer, that does not suffer from these disadvantages. The present invention is directed to this end.